Minimize or eliminate use of blank nodes

[ptgolden]

For a variety of reasons, it is undesirable to have blank nodes. @rybesh, you pointed out problems relating to:

• Error messages in the SHACL validator being unreadable when related to blank nodes

• Forming certain sorts of SPARQL queries

Additionally, if we take the approach in #44, it is impossible to refer to blank nodes in an annotation.

We use blank nodes to represent start/stop resources, and for referring to specific pages within sources. We can probably address both those cases by just giving those resources URIs based off the identifier of the associated period.